The countergravity, shell mold, casting process is particularly useful in the making of thin-wall castings and involves: sealing a bottom-gated shell mold, having a gas-permeable upper portion, (e.g., cope) to the mouth of a vacuum chamber such that the chamber encompasses the upper portion; immersing the underside of the mold in an underlying melt; and evacuating the chamber to draw melt up into the mold through one or more of the gates in the underside thereof. Such a process is shown in U.S. Pat. No. 4,340,108 wherein the mold comprises a resin-bonded-sand shell having cope and drag portions defining a molding cavity therebetween. Many castings made by such a process require the use of an expendable, retained core disposed within the mold cavity to shape the inside of the casting, such cores are engulfed by the melt, initially retained within the casting and finally removed as, for example, by disintegration. It is known to use hollow retained cores to reduce the amount of core material and to facilitate core removal.
Retained cores typically have a mounting extension on at least one end thereof which is anchored to the mold shell (i.e., usually at the parting line between the shell halves) to position the core in the molding cavity and support it against movement therein as the melt flows about it. Heretofore, the mounting extension has been simply buried deep within the material forming the mold shells, and, for thermally stable core materials (e.g., quartz), this is an acceptable way to mount the core. Such materials, however, are quite expensive especially in complicated shapes. Less expensive core materials such as resin-bonded-sand (e.g., hot-box, cold-box, shell etc.), or the like, on the other hand, can be formed into virtually any core shape desired and hence give the mold maker considerable flexibility. However, resin-bonded-sand core materials are thermally degradable in that the resin binder breaks down to form gases under the heat of the melt. With respect to such thermally-degradable, retained cores, we have found that the gases generated by the breakdown of the binder during casting is trapped by the surrounding metal and hence cannot escape the molding cavity through the walls of the gas-permeable shell walls. Rather, these trapped gases tend to become detrimentally occluded (e.g., as internal voids or surface pits) in the casting.
It is therefore an object of the present invention to provide an improved countergravity casting apparatus of the above-described type which is so constructed and arranged as to vent the breakdown gases generated by thermally-degradable, retained cores engulfed by metal within the molding cavity and thereby avoid occlusion of the gases in the finished casting. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.